Method and device for reducing engine noise

ABSTRACT

A method and device to reduce the noise produced by jet aircraft engines and/or other supersonic nozzles involves the use of corrugated engine seals ( 30 ) for the secondary internal divergent flaps ( 14 ) of the nozzle ( 10 ). Such corrugated seals ( 30 ) serve not only to eliminate shock-generated noise, but also generate a counter-rotating vorticity to force low speed mixing of surrounding ambient air with the high-speed jet exhaust. Lower exhaust velocities, combined with reduced levels of the turbulent Reynolds shear stress, lead to reduction of turbulence-generated noise, including Mach wave emission.

PRIORITY

This divisional application claims benefit to U.S. Non-Provisionalapplication No. 10/999,449, now allowed, filed Nov. 30, 2004 now U.S.Pat. No. 7,240,493, which claims priority to U.S. Provisional PatentApplication Ser. No. 60/525,912 filed Dec. 1, 2003, the entiredisclosure of which is incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with assistance from Grant NumbersN00014-02-1-0871 and N00014-02-1-0380 from the Office of Naval Research.The United States Government has rights to this invention.

FIELD OF THE INVENTION

The present invention relates to the field of jet aircraft enginesand/or other supersonic nozzles, and, more particularly, to a method anddevice to reduce the noise produced by jet aircraft engines and/or othersupersonic nozzles through the use of corrugated seals for the secondaryinternal divergent flaps of the nozzle. Furthermore, the presentinvention contemplates the use of prism-shaped extensions or chevronsattached to the primary outer flaps of the nozzle for further reductionof noise without any loss of aero-performance.

BACKGROUND OF THE INVENTION

The method and device of the present invention has particularapplicability for the jet engines of a military aircraft, such as theU.S. Navy's F/A-18 E/F Super Hornet aircraft in performance of the FieldCarrier Landing Practice (FCLP) mission profile. During performance ofthe FCLP mission profile, military aircraft, such as the F/A-18 E/F,operate with variable area engine nozzles which are scheduled to behighly overexpanded. This means that the nozzle exit static pressure issignificantly below the surrounding ambient pressure at the aircraft'saltitude above ground level. This overexpanded exhaust flow containsshocks in the exhaust plumes, the presence of which generates anefficient noise production mechanism known as “shock noise.” For afurther discussion of shock noise, reference is made to Seiner, J. M.,1984, “Advances in High-Speed Jet Aeroacoustics,” Invited Lecture, AIAAPaper No. 84-2275. This publication is incorporated herein by thisreference.

Furthermore, it is well-recognized that an overexpanded nozzle has alower aerodynamic performance efficiency than one that is fullyexpanded, i.e., where the exhaust static pressure equals the localaircraft ambient pressure. See Liepman, H. W., and Roshko, A., 1985,“Elements of Gasdynamics,” Dover Publications, Inc., Mineola, N.Y., apublication which is also incorporated herein by this reference. In anyevent, reduction of shock noise can generally be accomplished by designof the nozzle geometry to achieve fully expanded flow at the nozzleexit, where the exhaust static pressure is equal to ambient pressure.

In addition to shock noise, an additional efficient noise generatingmechanism is present within a supersonic exhaust regardless of whetherit contains shocks. This noise generating mechanism is referred to asMach wave emission. See Seiner, J. M., Bhat, T. R. S., and Ponton, M.K., 1994, “Mach Wave Emission From a High Temperature Supersonic Jet,”AIAA J., Vol. 32, No. 12, pp. 2345-2350, a publication which is alsoincorporated herein by this reference. To minimize this noise sourcerequires that the high-speed exhaust be forced to mix with the slowermoving surrounding air to achieve lower velocities in the exhaust plumethan would otherwise occur naturally. Lower exhaust velocities, combinedwith reduced levels of the turbulent Reynolds shear stress, lead toreduction of turbulence-generated noise, including Mach wave emission.

Accordingly, it would be desirable to provide a method and device tosubstantially reduce shock noise by providing a nozzle design andconstruction to achieve fully expanded flow at the nozzle exit, while atthe same time, generating the appropriate counter-rotating vorticity toforce low speed mixing of surrounding ambient air with the high-speedexhaust to reduce turbulence-generated noise.

SUMMARY OF THE INVENTION

The present invention is a method and device to reduce the noiseproduced by jet aircraft engines and/or other supersonic nozzles throughthe use of corrugated seals for the secondary internal divergent flapsof the nozzle.

In one exemplary embodiment of the present invention, each corrugatedseal has a cross-sectional shape of a truncated super ellipse of highaspect ratio with a circular quadrant extension from each side of thesuper ellipse to create a substantially horizontal portion at thesurface of the corrugated seal. After determining the nozzle areadistribution for shock-free flow at a particular power setting, thedifference between the original cross-sectional area of the nozzle atany given point along the length of the nozzle and the calculatedcross-sectional area for shock-free flow can be computed. Thisdifference is then divided by the number of corrugated seals to beinstalled. By making such a computation at discrete axial locationsalong the length of the nozzle, and assuming that the generalcross-sectional shape of the corrugated seal remains constant, atopological surface geometry for each corrugated seal is established.

Once these corrugated seals are installed in the nozzle, they serve notonly to eliminate shock-generated noise, but also generate acounter-rotating vorticity to force low speed mixing of surroundingambient air with the high-speed exhaust. Lower exhaust velocities,combined with reduced levels of the turbulent Reynolds shear stress,lead to reduction of turbulence-generated noise, including Mach waveemission.

Furthermore, the present invention contemplates the use of prism-shapedextensions or chevrons attached to the primary outer flaps of the nozzleto control thrust augmentation associated with corrugated seals andenhance the level of forced mixing for additional noise reductionwithout any loss of aero-performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional view of the exhaust nozzle of a jetaircraft engine;

FIG. 2 is a partial sectional view of the exhaust nozzle of a jetaircraft engine similar to that of FIG. 1, but illustrating the use ofcorrugated engine seals in accordance with the present invention;

FIG. 3 is a perspective view of an exemplary corrugated seal made inaccordance with the present invention;

FIG. 4 is a perspective view of the exemplary corrugated seal of FIG. 3;

FIG. 5 is a perspective view of an engine nozzle, including six of theexemplary corrugated seals of FIGS. 3 and 4;

FIG. 6 illustrates the general cross-sectional shape of the exemplarycorrugated seal of FIGS. 3 and 4;

FIG. 7 illustrates the general cross-sectional shape and positioning ofsix exemplary corrugated seals installed in an engine nozzle;

FIG. 8 chart illustrating the measured sound pressure level for a GEAEF404-402 engine at Mil-Pwr, N2=96.5% based on model testing,specifically comparing the measured sound pressure level with andwithout the corrugated seals of the present invention;

FIG. 9 is a chart illustrating the measured sound pressure level for aGEAE F404-402 engine at N2=95.0% based on model testing, specificallycomparing the measured sound pressure level with and without thecorrugated seals of the present invention;

FIG. 10 includes charts of nine acoustic narrow-band spectralcomparisons from model testing, specifically comparing the measuredsound pressure level for a GEAE F404-402 engine at Mil-Pwr, N2=96.5%with and without the corrugated seals of the present invention;

FIG. 11 includes charts of nine acoustic narrow-band spectralcomparisons from model testing, specifically comparing the measuredsound pressure level for a GEAE F404-402 engine at N2=95.0% with andwithout the corrugated seals of the present invention;

FIG. 12 is a representation of the infrared image for a nozzle withoutthe corrugated seals of the present invention at Mil-Pwr, N2=96.5% inmodel testing;

FIG. 13 is a representation of the infrared image of a nozzle with thecorrugated seals of the present invention at Mil-Pwr, N2=96.5% in modeltesting;

FIG. 14 is a chart illustrating the measured sound pressure level (SPL)for a GEAE F404-400 engine at N2=95.5% based on actual engine testing,specifically comparing the measured sound pressure level with andwithout the corrugated seals of the present invention;

FIG. 15 is a chart illustrating narrow-band acoustic spectra associatedwith noise emission angles of 40° and 140° for a GEAE F404-400 engine atN2=95.5% based on actual engine testing;

FIG. 16 is another chart illustrating narrow-band acoustic spectraassociated with noise emission angles of 40° and 140° for a GEAEF404-400 engine at N2=95.5%, but in this case, the corrugated seals ofthe present invention are used along with twelve chevrons;

FIG. 17 is a representation of the infrared image for a nozzle withoutthe corrugated seals of the present invention at N2=95.5% in actualengine testing;

FIG. 18 representation of the infrared image of a nozzle with thecorrugated seals of the present invention at N2=95.5% in actual enginetesting;

FIG. 19 is a perspective view of an exemplary chevron; and

FIG. 20 is a perspective view illustrating the attachment of twelvechevrons to the outer divergent flaps of a nozzle, which alsoincorporates the corrugated seals of the present invention.

DESCRIPTION OF THE INVENTION

The present invention is a method and device to reduce the noiseproduced by jet aircraft engines and/or other supersonic nozzles throughthe use of corrugated seals for the secondary internal divergent flapsof the jet aircraft engine. Furthermore, the present inventioncontemplates the use of prism-shaped extensions or chevrons attached tothe primary outer flaps of the nozzle to control thrust augmentationassociated with corrugated seals and enhance the level of forced mixingfor additional noise reduction without any loss of aero-performance.

FIG. 1 is a partial sectional view of the exhaust nozzle 10 of a jetaircraft engine. Such an exhaust nozzle 10 defines a flowpath, asindicated by the arrows in FIG. 1, for engine exhaust gas which exitsgenerally horizontally from an aft end of an aircraft. The nozzle 10generally includes a converging portion and a diverging portiondownstream from the converging portion. As gas moves through the nozzle10, from the converging portion through the diverging portion, thecross-sectional area available for gas flow decreases in the convergingportion and increases in the diverging portion. A plane of minimumcross-sectional area, known as a “throat” is located between theconverging portion of the nozzle 10 and the diverging portion.

Of particular importance to the present invention, the nozzle 10includes circumferentially spaced primary outer divergent flaps 12,which are pivotable to alter the cross-sectional area as the gas exitsthe nozzle 10. Furthermore, the nozzle 10 includes secondary internaldivergent flaps 14. The relationship and positioning of the respectiveflaps 12, 14 is further illustrated in FIG. 5. In any event, such aconstruction of the exhaust nozzle 10 of a jet aircraft engine iswell-known and understood by one of ordinary skill in the art.

FIG. 2 is a partial sectional view of the exhaust nozzle 10 of a jetaircraft engine similar to that of FIG. 1, but illustrating the use ofcorrugated engine seals 20 in accordance with the present invention.Specifically, such corrugated engine seals 20 replace a predeterminednumber of the standard seals for the secondary internal divergent flaps14 of the jet aircraft engine 10. For example, in the exemplaryembodiment illustrated in FIG. 5, there are six corrugated engine seals20 and six standard seals 30. In other words, every other standard seal30 has been replaced by a corrugated engine seal 20 made in accordancewith the present invention.

In determining the appropriate geometry for these corrugated engineseals 20 to maximize their ability to reduce noise, first, it isimportant to identify the pertinent aircraft altitudes with engine powersettings that define the mission profile. Furthermore, aircraftconfiguration, such as wing loading, landing gear position, and flapposition, are important for proper determination of aircraft speed at aparticular power setting. As mentioned above, the present invention hasparticular applicability for the jet engines of a supersonic aircraft,such as the U.S. Navy's F/A-18 E/F Super Hornet aircraft in performanceof the Field Carrier Landing Practice (FCLP) mission profile. Thisaircraft uses F404-402 engines manufactured by General Electric AircraftEngines (“GEAE”) Cincinnati, Ohio.

Then, certain operating parameters of the aircraft and enginesassociated with the defined mission profile must be determined. This canbe achieved through the running of an installed engine cycle deck forthe engine, which is a simulation that generates aerothermal numericalrepresentations to characterize engine performance. Often, engine cycledecks for specific engines are available from the engine manufacturer orthe U.S. Government Federal Laboratory responsible for a militaryaircraft's mission.

In any event, the most effective noise reduction design is oftenassociated with the maximum engine thrust for the particular mission.The maximum thrust for the U.S. Navy's F/A-18 E/F Super Hornet or othermilitary aircraft is commonly referred to as “Military Engine Power” or“Mil-Pwr.” From the installed engine cycle deck, the engine nozzlepressure ratio, exit static pressure, engine total temperature aftermixing with the fan engine flow, engine weight flow, throat area, nozzleexit area, and ambient pressure and temperature at altitude aredetermined at Mil-Pwr. Isentropic equations, such as those described inLiepman, H. W., and Roshko, A., 1985, “Elements of Gasdynamics”, DoverPublications, Inc., Mineola, N.Y. (referenced above), can then be usedto compute the nozzle exit Mach number and Mach number for fullyexpanded flow where the exit static pressure matches that of ambientpressure.

After determining the operating parameters at Mil-Pwr or another powersetting of interest, a Method of Characteristics (MOC) solution isobtained based on the actual throat area and an exit area that thatproduces fully expanded flow for the nozzle pressure ratio and totaltemperature as determined from the engine cycle deck. For engine nozzleswith standard, substantially flat seals, a high-order polygonal exitarea is assumed. For example, for the GEAE F404-402 engines referencedabove, a twelve-sided exit area is assumed. MOC nozzle codes aregenerally available from various sources, including U.S. GovernmentFederal Laboratories.

The MOC solution provides an area distribution from the throat of thenozzle to exit of the nozzle that produces shock-free flow. In otherwords, the MOC solution establishes a optimal cross-sectional area ataxial locations along the length of the nozzle for producing shock-freeflow. Of course, this area distribution includes values that are alwayssmaller than the existing nozzle area distribution if the flow wasoverexpanded, and values that are greater if the flow was underexpanded.As mentioned above, military aircraft generally have variable areaengine nozzles which are scheduled to be highly overexpanded.

Once the optimal area distribution from the throat of the nozzle to exitof the nozzle has been determined through the MOC solution, theappropriate geometry and dimensions for the corrugated seals 20 can bedetermined. Referring now to FIGS. 3-7, it has been determined that anappropriate cross-sectional shape for a corrugated seal 20 made inaccordance with the present invention is that of a truncated superellipse of high aspect ratio with a circular quadrant extension fromeach side of the super ellipse to create a substantially horizontalportion at the surface of the corrugated seal 20, which facilitatesattachment to a nozzle. This general shape is perhaps best illustratedin FIG. 6, with the truncated super ellipse portion being identified byreference numeral 22, and the respective circular quadrant extensionsbeing identified by reference numerals 24 a and 24 b. Such across-sectional shape produces strong counter-rotating vorticity for thedesired forced mixing of high-speed with low-speed flow. Furthermore,and referring still to FIG. 6, through experimentation, it has beendetermined that an appropriate aspect ratio for the super ellipse isapproximately 4:1 (H v. W), which, in the exemplary embodimentillustrated in FIGS. 3-7, is substantially constant along the length ofthe nozzle.

In any event, the numerical dimensions of the corrugated seals 20 areselected so as to provide the area distribution required to produceshock-free flow. In other words, and as mentioned above, the MOCsolution establishes a optimal cross-sectional area at axial locationsalong the length of the nozzle for producing shock-free flow. Thus, thepenetration of the corrugated seals 20 into the exhaust flow (i.e., theheight of the seals) is determined by the difference in areadistribution of the original nozzle compared to the calculated areadistribution for shock-free flow.

Specifically, after having determined the area distribution forshock-free flow at a particular power setting (e.g., Mil-Pwr), thedifference between the original cross-sectional area of the nozzle atany given point along the length of the nozzle and the calculatedcross-sectional area for shock-free flow can be computed. Thisdifference is then divided by the number of corrugated seals 20 to beinstalled. As mentioned above, in the exemplary embodiment illustratedin FIG. 5, there are six corrugated engine seals 20. Accordingly, thedifference in cross-sectional area would be divided by six. For example,if the original cross-sectional area was 240 square inches at a givenaxial location, and the MOC solution indicated that the optimalcross-sectional area at this axial location for producing shock-freeflow was 210 square inches, the difference would be 30 square inches.Dividing by the number of seals, six, would indicate that thecross-sectional area for a corrugated seal at this axial location shouldbe five square inches.

By making such a computation at discrete axial locations along thelength of the nozzle, and assuming that the general cross-sectionalshape of the corrugated seal 20 remains constant, a topological surfacegeometry for the corrugated seal 20 is established. In other words, theslope or contour of the corrugated seal 20 along its length isestablished, perhaps as best illustrated in FIGS. 3 and 4. As mentionedabove, in addition to elimination of shock noise, the shape of thecorrugated seals 20 generates counter-rotating vorticity to force lowspeed mixing of surrounding ambient air with the high-speed exhaust.Lower exhaust velocities, combined with reduced levels of the turbulentReynolds shear stress, lead to reduction of turbulence-generated noise,including Mach wave emission.

Therefore to the extent that the method of the present invention isapplied to a jet engine design, it may be generally characterized asincluding the following steps: (1) identifying a mission profile andpower setting of interest for the engine; (2) determining certainoperating parameters of the engine based on the mission profile; (3)obtaining a Method of Characteristics (MOC) solution based on thecertain operating parameters that produces fully expanded flow; (4)determining an appropriate geometry and dimensions for a predeterminednumber of corrugated seals based on the MOC solution; and (5) installingsuch corrugated seals in the engine nozzle.

Model Testing

To confirm the efficacy of the method and device of the presentinvention as described above, 1/10th scale model testing of thecorrugated seal geometry was conducted for certain power settingsidentified in the engine cycle deck for the F404-402 enginesmanufactured by General Electric Aircraft Engines of Cincinnati, Ohio.

FIG. 8 is a chart illustrating the measured sound pressure level (SPL)for the F404-402 engine at Mil-Pwr, N2=96.5% (Point 10S of the enginecycle deck) without the corrugated seals of the present invention(“baseline”) and with six corrugated seals as described above withreference to FIGS. 3-7. As FIG. 8 demonstrates, there is an appreciablereduction in the SPL at each measurement location when the corrugatedseals are used. Specifically, there is as much as a 3 dB reduction inthe rear arc (90-180°), and a 5 dB reduction in the forward arc (0-90°).

FIG. 9 is a chart illustrating the measured sound pressure level (SPL)for the F404-402 engine at N2=95.0% (Point 7S of the engine cycle deck)without the corrugated seals of the present invention (“baseline”) andwith six corrugated seals as described above with reference to FIGS.3-7. Again, there is an appreciable reduction in the SPL at almost everymeasurement location when the corrugated seals are used.

FIG. 10 includes charts of nine acoustic narrow-band spectralcomparisons, specifically comparing the measured sound pressure levelfor the F404-402 engine at Mil-Pwr, N2=96.5% without the corrugatedseals of the present invention (“baseline”)and with six corrugatedseals, as described above with reference to FIGS. 3-7. The acousticnarrow-band spectral comparisons are for inlet angles from 45° to 160°.As the charts of FIG. 10 demonstrate, the baseline spectra containsignificant shock noise content as exhibited by the spectral shown near10⁴ hertz at angles of 45-90°. However, all traces of such shock noiseare removed through use of the corrugated seals of the presentinvention. At angles between 90° and 180°, turbulence-generated or jetmixing noise dominates the acoustic spectrum. However, some noisereduction is still observed at these angles due to the selection of thesuper ellipse shape for the corrugated seal. As described above, such across-sectional shape produces strong counter-rotating vorticity for thedesired forced mixing of high-speed with low-speed flow.

FIG. 11 includes charts of nine acoustic narrow-band spectralcomparisons, specifically comparing the measured sound pressure levelfor the F404-402 engine at N2=95.0% without the corrugated seals of thepresent invention (“baseline”) and with six corrugated seals, asdescribed above with reference to FIGS. 3-7. Similar to the resultsdescribed above with reference to FIG. 10, all traces of shock noise areremoved through use of the corrugated seals of the present invention.Furthermore, there is a reduction of turbulence-generated or jet mixingnoise.

Finally, as part of the model testing, a study was conducted todetermine the potential for reduction of the exhaust plume infrared (IR)emission. In this regard, a short wave imaging radiometer was used toimage the exhaust plume with and without the corrugated seals. FIG. 12is a representation of the IR image for a baseline nozzle without thecorrugated seals at Mil-Pwr, N2=96.5%, whereas FIG. 13 is arepresentation of the IR image of the nozzle with six corrugated sealsat Mil-Pwr, N2=96.5%. Comparing FIGS. 12 and 13, it becomes apparentthat there is an appreciable reduction in emissions. Achieving reducedinfrared emissions is especially important for military applicationssince the major weapon used against military aircraft, such as the U.S.Navy's F/A-18 E/F Super Hornet, is a heat-seeking missile. Reducedinfrared emissions decreases the probability that the missile will beable to lock-on and destroy the aircraft. In this regard, referringagain to FIGS. 12 and 13, infrared radiance is proportional to thefourth power of the plume temperature. Accordingly, a heat-seekingmissile would have to approach the exhaust plume at much reduceddistances to lock-on, thereby increasing the probability for successfulevasive maneuver by the targeted aircraft.

Engine Testing

For further confirmation of the efficacy of the method and device of thepresent invention as described above, testing of a F404-400 enginemanufactured by General Electric Aircraft Engines of Cincinnati, Ohiowas conducted at the Naval Air Warfare Center Aircraft Division atLakehurst, N.J. (NAWCADLKE). For purposes of this testing, thecorrugated seals were designed for N2=95.5% (Point 8S of the enginecycle deck).

FIG. 14 is a chart illustrating the measured sound pressure level (SPL)for the F404-400 engine at N2=95.5% (Point 8S of the engine cycle deck)at discrete angles ranging from 0° to 180° relative to the engine inletaxis. Specifically, this chart compares the SPL without the corrugatedseals of the present invention (“baseline”) and with six corrugatedseals as described above with reference to FIGS. 3-7. As FIG. 14demonstrates, consistent with the above-described model testing, thereis an appreciable reduction in the SPL at each measurement location whenthe corrugated seals are used. Specifically, for the forward arc angles(0-90°), the corrugated seals of the present invention significantlyreduce noise due to elimination of shock noise, and the corrugated sealsare also effective at reducing noise in the rear arc (90-180°) that isdominated by turbulence-generated or jet mixing noise. Furthermore,although not illustrated in the Figures, aero-performance measurementsindicate that the use of the corrugated seals at N2=95.5% resulted in anadditional 50 pounds of thrust. In other words, any effect onaero-performance was negligible.

FIG. 15 is a chart illustrating narrow-band acoustic spectra associatedwith noise emission angles of 40° and 140° for the F404-400 engine atN2=95.5%. The level of acoustic suppression resulting from use of thecorrugated seals of the present invention is similar to that recorded inthe model testing for both shock noise and turbulence-generated noise.

Furthermore, as part of the engine testing at NAWCADLKE, engine testingwas also conducted using prism-shaped extensions, known as chevrons,attached to the nozzle and extending into the exhaust stream to achievegreater levels of forced mixing of the high-speed exhaust with theslower moving surrounding air. An exemplary embodiment of such a chevron40 is illustrated in FIG. 19, and FIG. 20 illustrates the attachment oftwelve (12) such chevrons 40 to the outer divergent flaps 12 of a nozzle10. Such use of prism-shaped extensions or chevrons is generally knownthe art. See, eg., Grosch, C. E., Seiner, J. M., Hussani, M. Y., andJackson, T. L., 1997, “Numerical Simulation of Mixing Enhancement In aHot Supersonic Jet”, Physics of Fluids, Vol. 9, Part 4, pp. 1125-1143, apublication which is incorporated herein by this reference. The chevrons40 are essentially inverted Delta-Wings, whose planforms are noted forgeneration of high-lift or strong counter-rotating vorticity. Thespacing of the chevrons 40 is critical for enhanced mixing and theirprojected area into the flow is related directly to performance loss,thus generally limiting their size to less than 1% of the nozzle exitarea.

Referring again to FIG. 14, the chart illustrates that the measuredsound pressure level (SPL) for the F404-400 engine at N2=95.5% usingtwelve chevrons is only improved in the rear arc (i.e., at inlet anglesexceeding 90°) where the noise is dominated by turbulence-generated orjet mixing noise. Indeed, the use of the chevrons caused an increase innoise in the forward arc. In short, although the corrugated seals of thepresent invention and chevrons are effective at reducing noise in therear arc that is dominated by turbulence generated or jet mixing noise,only the corrugated seals are effective in reducing noise in the forwardarc because of their ability to reduce shock noise.

FIG. 16 is another chart illustrating narrow-band acoustic spectraassociated with noise emission angles of 40° and 140° for the F404-400engine at N2=95.5%, but in this case, the corrugated seals of thepresent invention are used along with twelve chevrons, as illustrated inFIG. 20. As the chart demonstrates, although the higher acoustic energyat 140° was significantly reduced, shock noise was increased.

Finally, similar to the model testing, a study was conducted todetermine the potential for reduction of the exhaust plume infrared (IR)emission. FIG. 17 is a representation of the IR image for a baselinenozzle without the corrugated seals at N2=95.5%, whereas FIG. 18 is arepresentation of the IR image of the nozzle with six corrugated sealsat N2=95.5%. Comparing FIGS. 17 and 18, it again becomes apparent thatthere is an appreciable reduction in emissions. As described above withreference to FIGS. 12 and 13, reduced infrared emissions decrease theacquisition time for missile lock-on, and consequently, reduce theprobability that the missile will be able to lock-on and destroy theaircraft.

One of ordinary skill in the art will also recognize that additionalembodiments and/or implementations are possible without departing fromthe teachings of the present invention or the scope of the claims whichfollow. This detailed description, and particularly the specific detailsof the exemplary embodiments and testing configurations disclosedtherein, is given primarily for clarity of understanding, and nounnecessary limitations are to be understood therefrom, formodifications will become obvious to those skilled in the art uponreading this disclosure and may be made without departing from thespirit or scope of the claimed invention.

1. A method for reducing noise produced by a jet engine having a nozzlewith secondary internal divergent flaps, comprising the steps of:identifying a mission profile for the engine by identifying pertinentaircraft altitudes with engine power settings; determining certainoperating parameters of the engine based on the mission profile;obtaining a Method of Characteristics (MOC) solution based on thecertain operating parameters that produces fully expanded exhaust flow;determining an appropriate geometry and dimensions for a predeterminednumber of corrugated seals for the secondary internal divergent flaps ofthe engine based on the MOC solution; and installing such corrugatedseals in the engine nozzle.
 2. The method as recited in claim 1, inwhich the step of determining certain operating parameters of the engineis achieved through a simulation that generates aerothermal numericalrepresentations to characterize engine performance.
 3. The method asrecited in claim 1, in which the step of determining the appropriategeometry and dimensions for the predetermined number of corrugated sealsfirst assumes that an appropriate cross-sectional shape for eachcorrugated seal is generally that of a truncated super ellipse with acircular quadrant extension from each side of the super ellipse.
 4. Themethod as recited in claim 3, in which the super ellipse has a highaspect ratio.
 5. The method as recited in claim 4, in which the aspectratio of the super ellipse is approximately 4:1.
 6. The method asrecited in claim 1, and further comprising the step of: attaching apredetermined number of chevrons to the engine nozzle, each such chevronextending into the exhaust flow to achieve greater levels of forcedmixing of high-speed exhaust with slower moving surrounding air.
 7. Themethod of claim 1, wherein the mission profile is a Field CarrierLanding Mission Profile.
 8. A method for reducing noise associated witha supersonic nozzle, comprising the steps of: determining certainoperating parameters of the supersonic nozzle; determining an areadistribution from the throat of the supersonic nozzle to the exit of thesupersonic nozzle based on the certain operating parameters thatproduces fully expanded exhaust flow; determining an appropriategeometry and dimensions for a predetermined number of corrugated sealsfor the supersonic nozzle based on the area distribution that producesfully expanded flow; and installing such corrugated seals in thesupersonic nozzle.
 9. The method as recited in claim 8, in which thestep of determining the appropriate geometry and dimensions for thepredetermined number of corrugated seals first assumes that anappropriate cross-sectional shape for each corrugated seal is generallythat of a truncated super ellipse with a circular quadrant extensionfrom each side of the super ellipse.
 10. The method as recited in claim9, in which the super ellipse has a high aspect ratio.
 11. The method asrecited in claim 10, in which the aspect ratio of the super ellipse isapproximately 4:1.
 12. A method for reducing infrared emissions producedby a jet engine having a nozzle with secondary internal divergent flaps,comprising the steps of: identifying a mission profile for the engine byat least identifying pertinent aircraft altitudes with engine powersettings; determining certain operating parameters of the engine basedon the mission profile; obtaining a Method of Characteristics (MOC)solution based on the certain operating parameters that produces fullyexpanded exhaust flow; determining an appropriate geometry anddimensions for a predetermined number of corrugated seals for thesecondary internal divergent flaps of the engine based on the MOCsolution; and installing such corrugated seals in the engine nozzle. 13.The method of claim 12, wherein the mission profile is a Field CarrierLanding Mission Profile.